Nitroarences and especially nitropyrenes are extremely powerful bacterial mutagens. These chemicals are widely distributed in the environment as the result of the facile ambient nitration of polycyclic aromatic hydrocarbons orginating from incomplete combustion processes. We have shown that the mutagenicity of nitropyrences is dependent upon (a) biotransformation to ultimatic mutagens (hydroxamic acid esters) and (b) DNA adduct formation and (c) the error-free or erro-prone repair of these. It is proposed to investigate these processes. Evidence has already been obtained that reductive activation of the nitropyrences is carried out by specific nitro-reductases and indications have been obtained that use could be made of these to identify nitropyrenes in environmental mixtures. It is proposed to isolate and characterize nitroreductase-defincient microorganisms as well as nitroreductase-proficient mutants and using these to study the "activation" of nitroarenes to mutagens, the chemical nature of the DNA adducts and their excision from the target cell. And finally to assess the usefulness of these nitroreductase mutants to specifically identify nitroarenes in environmental mixtures.